A general theory is developed to analyze the efficiency and income distribution effects of a biofuel consumer tax credit and the interaction effects with a price contingent farm subsidy. Using the U.S. ethanol market as a stylized example, ethanol prices rise above the gasoline price by the amount of the tax credit. Corn farmers therefore gain directly while gasoline consumers only gain from any reduction in world oil prices due to the extra ethanol production and domestic oil producers lose. Because increased ethanol production improves the terms of trade in both the export of corn and the import of oil, we determine the optimal tax credit and the conditions affecting it. Historically, the intercept of the ethanol supply curve is above the gasoline price. Hence, part of the tax credit is redundant and represents ‘rectangular’ deadweight costs. The tax credit reduces the tax costs of price supports but incurs tax costs itself and increases consumer costs of corn. Price supports eliminate, create, have no effect or have an ambiguous effect on rectangular deadweight costs, depending on whether there is ex ante or ex post water in the tax credit. There are situations where ethanol production occurs only because of price supports. A stylized empirical model of the U.S. corn market is calibrated to illustrate the welfare effects of a tax credit. Net social costs of the tax credit averaged $683 million. Rectangular deadweight costs averaged $1,056 mil., more than offsetting the improved terms of trade and reduced price contingent farm subsidies, and representing over 50 percent of the tax cost of the tax credit.
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